There are two types of finished edge which may be achieved after sharpening a blade, a rough finish and a fine finish. Both rough and fine finishes result in a sharper blade, but a fine finish is preferred. The sharpening methods which result in these two finishes are different, being properly employed under different circumstances.
The grinding method of sharpening is used to achieve the rough finish. It is a desirable method when for example, the blade is chipped, seriously dulled, or when a "belly" or rounding has formed. The grinding method works by removing metal from the blade by abrading against it, thereby exposing a new cutting surface. This method is not desirable when the blade has been only slightly dulled through use, since it is unnecessary to expose a new cutting surface.
The fine finish is achieved by using what is termed a steel. A steel is an elongated tubular piece of hardened metal against which a blade is drawn. The steel works by pushing against the metal of the blade, with the effect of moving back the metal at the edge. Use of a steel does not abrade the blade or wear away at the metal. The angle at which the blade is drawn against the steel and the pressure used in drawing it will be a proportional function of thickness of the blade and dullness of its edge.
The manual sharpening of blades using either method requires a great deal of dexterity. The risk of being cut while attempting to sharpen a knife manually has always been great. To that end, mechanical sharpening devices have been developed for achieving rough finishes, however, no mechanical steel has been invented for achieving fine finishes until the present invention.
The mechanical sharpening devices known in the art work on the grinding principle. Representative is the honing machine of Battocchi (U.S. Pat. No. 3,526,061) wherein two grinding wheels, separated by a blade guide block, rotate in opposite directions; upwardly at their junction with the guide block. The blade to be sharpened is drawn alternatively between one side of the guide block and the grinding wheel and then the other. While Battocchi claims to provide "an exceptionally keen, precision finished cutting edge," that edge would be considered rough finished, according to the terms defined above. Various other configurations, adapted to sharpen both sides of the blade at once, such as the patents to Murchison (U.S. Pat. No. 2,795,156), to Dilg (U.S. Pat. No. 813,142), and to Judd (U.S. Pat. No. 1,474,636); or adapted to sharpening scissors, such as Gangelhoff (U.S. Pat. No. 4,048,760), have been developed, but each involves grinding and results in a rough finish. U.S. Pat. No. 2,707,852 to Fillweber shows a grinding machine where two grind wheels are overlapping. Again, the critical distinction is that this involves grinding. The need for an easy-to-use mechanical device and method for achieving a fine finish has remained unsatisfied until the invention of the present mechanical steel.